This invention relates to an electromagnetic delay line for delaying the luminance signal in a color television receiver and more particularly to a dual section distributed parameter delay line.
In color television receiver circuitry, the luminance portion of the picture signal must be delayed typically by a little less than a microsecond so that it arrives at the picture tube simultaneously with the chrominance signal. A great variety of lumped-parameter and of distributed parameter delay lines have been used for this purpose.
However, in recent years certain changes and improvements have been made in other portions of TV receivers which generate relatively large magnetic fields within the receiver. For example, switching-mode high voltage power supplies have been introduced to reduce power consumption. The output transformer in these power supplies radiate electromagnetic energy at a fundamental frequency as high as 35 KHz.
Another source of stray magnetic fields within color TV receivers is the deflection coil. There is a trend toward the use of so-called precision-in-line (P.I.L.) picture tubes which call for toroidal deflection yokes in place of the saddle type yokes used earlier. Experience has shown that the toroidal yokes generate stronger stray fields. These signals usually have a fundamental frequency of approximately 15 KHz.
Manufacturers of these modern color TV receivers have found that the large stray fields induce unwanted noise voltages into the delay-line. Among other adverse effects on the performance of the receiver, the black level is seriously altered, the relative position of the delay-line within the receiver is critical which complicates initial tune-up and field adjustments, and visible disturbances of the picture occur.
Several manufacturers have begun to use shielded delay-lines. The shield usually consists of two halves of a ferro-magnetic material placed around the delay-line, the halves being connected together and to a ground point in the receiver. Besides the additional costs, such shielded delay-lines are substantially larger and heavier than their non-shielded counterparts.
It is an object of the present invention to provide an improved delay line that overcomes the above noted problem.